fix(web-ui): prevent ACP session hydrate deadlock on re-switch

[xielixing]

Problem

When switching back to a previously-loaded ACP session, the UI shows a permanent loading overlay and never displays content. The agent appears stuck.

Issue: #1038

Root Cause

A deadlock in switchChatSession (SessionModule.ts) for ACP sessions:

1. shouldHydrateHistoricalSessionBeforeSwitch returns true for isHistorical sessions with no renderable content
2. shouldActivateBeforeHydrate requires consumeRecentHistorySessionOpenIntent(sessionId) — but running ACP sessions never get an intent (they're filtered by isRunning in SessionsSection.tsx)
3. With shouldActivateBeforeHydrate=false, the flow is hydrate-first-then-activate (lines 700-728)
4. hydrateHistoricalSession defaults deferFullHistoryUntilActive=true (line 256)
5. When hydrate completes, activeSessionId !== sessionId is still true (activation hasn't happened yet) → skipStaleLocalHydrateCommit in FlowChatStore.ts:7398-7401 fires and discards the loaded turns, resetting to metadata-only
6. Then switchSession(sessionId) activates it (line 728) — but with empty turns and metadata-only state, the overlay is stuck forever

Fix

Pass deferFullHistoryUntilActive: false for ACP sessions in the hydrateHistoricalSession call within switchChatSession. This ensures the hydrate commit is not discarded even when activation happens after hydration. The subsequent switchSession call then activates a session that already has its loaded turns.

Changes:

• Import isAcpFlowSession from ../../utils/acpSession
• Pass deferFullHistoryUntilActive: !isAcpFlowSession(session) to the hydrate call

Non-ACP sessions retain the default true behavior (defer until active), so this change has no effect on existing session switching behavior.

Testing

• TypeScript compilation passes (no new type errors)
• Existing session switching behavior for non-ACP sessions is unchanged
• ACP sessions now properly commit their loaded history even when activation follows hydration